(1) Field of the Invention
This invention relates to an optical transmission system for transmitting frequency division multiplex signals used for the CATV system and other systems.
(2) Description of the Related Arts
According to the CATV system, multichannel TV signals which have been frequency division multiplexed are converted into optical signals and transmitted to a receiver via a single transmission line. The receiver, on the other hand, distributes received optical signals into different channels to demodulate.
The CATV system utilizes an optical transmission system because this permits the use of a laser diode (hereinafter referred to as LD) as an optical converter. An LD, which has very low distortion factor attributable to its good linearity, can perform excellent transmission as well as reducing carrier-to-noise ratio (hereinafter referred to as CNR). Such feature of LDs is described in "80-Channel AM-FDM TV Signal Optical Transmission System" by Tanabe et. al. in "National Technical Report Vol. 36, No. 6, Des. 1990", and "TV channel capacity of lightwave multichannel AM SCM systems as limited by laser threshold nonleniarity" pp.18-19 by C. J. Chung and I. Jacobs in Optical Communication Conference, March 1992. The latter-mentioned document says that LDs have excellent distortion characteristics even when multiplexed signals whose total sum of light modulation indexes is more than 1 are transmitted.
Nowadays, providing a much larger number of channels in the CATV system has been proposed to offer a wider range of services.
If VSB-AM signals of an analog modulation system, which have been already used for the present channels, are further used for channels to be added, the total sum of the levels of all the VSB-AM signals will exceed permissible amplitude, or a linear region of LDs as a light converter because these signals are susceptible to noises when required to raise their levels. Thus, the number of channels able to be added is limited; if LDs are already used for almost the entire amplitude for the present channels, no more channel can be added in such a system.
A proposal has been made to realize that as many channels as desired can be added if signals modulated by M-ary Quadrature Amplitude Modulation (hereinafter referred to as M-QAM) are used for the additional channels and then these M-QAM signals are frequency division multiplexed together with VSB-AM signals for the present channels. Such idea is disclosed, for example, in "Multichannel M-QAM for CATV Distribution" WB4 pp. 21-22 by I. M. I. Habbab, LEOS Summer Topical Meeting Digest, Wednesday, July 29, 1992.
The reason that the use of the M-QAM signal permits an increase in number of the channels as desired is as follows:
QAM signals express information to be transmitted both by amplitude and phases, which enables the information to be transmitted with a small amplitude. In addition, QAM is a type of a digital modulation system in which M-divided information is expressed, so that the signal is harder to be affected by noises than those of the analog modulation system, thus performing excellent transmission with small CNR.
A theoretical analysis between the CNR of QAM signals and its error rate is given in "Digital Communications" MacGraw-Hill Series in Electrical Engineering Second Edition, pp. 278-285 by J. G. Proakis. According to this book, maintaining a predetermined error rate automatically determines a CNR. Therefore, when it is desired to increase the number of channels in the CATV system, the device of each channel can be designed so that a CNR to maintain an error rate required as a system meets the CNR of J. G. Proakis. Since a CNR corresponds to the light modulation index of the signal, the modulation index of the modulator can be adjusted to a predetermined error rate.
However, the inventors of this invention actually multiplexed AM signals together with 16-QAM signals and experimentally transmitted them under the condition that the total sum of their light modulation indexes is beyond 1. And it was revealed through the experiment that the error rate of the 16-QAM signals had no such theoretical relation with a CNR, or a light modulation index as J. G. Proakis says, and that Guaranteeing the required error rate demands to increase the levels of the QAM signals so as to obtain quite a large SNR or light modulation index. Accordingly, to increase the levels of QAM signals sets limits to the number of channels being added as long as the amplitude of LDs is not changed.